The Connection Between Forests, Carbon, and Climate Change

According to a recent study from Oregon State University, leaves in forest canopies cannot lower their internal temperatures below the ambient temperature. This suggests that the capacity of trees to prevent harmful temperature increases and absorb carbon dioxide will be compromised in a warmer, drier climate.

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The findings of an international effort concerning scientists from numerous universities and government agencies contradict the widely held scientific concept that canopy leaves can maintain their temperature in an ideal range for photosynthesis - the process by which green plants make their food from sunlight and carbon dioxide.

According to lead author Chris Still of the OSU College of Forestry, whose research was published today in the Proceedings of the National Academy of Sciences, many of the world’s forests may be nearing their thermal limit for carbon uptake. This research is crucial for recognizing and anticipating how plants will respond to climate change.

A hypothesis known as limited leaf homeothermy argues that through a combination of functional traits and physiological responses, leaves can keep their daytime temperature close to the best temperature for photosynthesis and below what is damaging for them.

Chris Still, Study Lead Author, College of Forestry, Oregon State University

“Specifically, leaves should cool below air temperature at higher temperatures, typically greater than 25 or 30 degrees Celsius. That theory also implies that the impact of climate warming on forests will be partially mitigated by the leaves’ cooling response,” Still said.

From the Panamanian rain forest to the high-elevation tree line in Colorado, Still and colleagues used thermal imaging to examine the canopy-leaf temperature. They discovered that, contrary to what the restricted leaf homeothermy theory anticipated, canopy leaves do not continuously cool below daytime air temperatures or remain within a narrow range of temperatures.

The thermal cameras were fixed to towers with instruments for measuring various environmental factors, including carbon, water, and energy “fluxes” — exchanges between the forest and atmosphere.

“Using high-frequency, continuous thermal imaging to monitor forest canopies changes what we can learn about how forests are dealing with the stress of rising temperatures,” said Andrew Richardson, a professor at Northern Arizona University and a co-author of the study.

“Before thermal cameras, if you wanted to measure canopy temperature you had to stick thermocouples to leaves with Band-Aids and wait until the wind pulled them off. But these cameras let us measure change 24 hours a day, seven days a week, across many seasons and years.”

The study revealed that canopy leaves heat up more quickly than the surrounding air, stay warmer for most of the day, and only cool below air temperature in the middle to late afternoon.

According to scientists, increased canopy leaf temperatures due to future global warming are anticipated to have a detrimental influence on forest carbon cycling and will increase the probability of forest mortality.

“Leaf temperature has long been recognized as important for plant function because of its influence on carbon metabolism and water and energy exchanges. If canopy photosynthesis declines with increasing temperature, the ability of forests to act as a carbon sink will be reduced,” Still said.

According to Still, how leaf size fluctuates with climate and latitude and canopy structure impacts leaf temperature in various ecosystems. Large leaves are more common in warm, humid settings, while smaller leaves and higher reflectance, which improve the ability to shed heat and increase cooling, are more common in plants that grow in hot, dry conditions.

The positive net photosynthesis criteria — the carbon fixation rate minus the amount of carbon dioxide released during plant respiration — are already being approached or exceeded in a large portion of the warm, wet tropics.

“If leaves are generally warmer than the surrounding air, as our findings suggest, trees may be approaching critical thresholds of temperature stress faster than we expect,” Richardson believed.

“Our results have big implications for understating how plants acclimate to warming, and they suggest a limited ability for canopy leaves to regulate their temperature. Our data and analyses suggest a warming climate will result in even higher canopy leaf temperatures, likely leading to reduction of carbon assimilation capacity and eventually heat damage,” Still added.

The National Science Foundation funded this investigation.

Journal Reference

Still, C. J., et al. (2022) No evidence of canopy-scale leaf thermoregulation to cool leaves below air temperature across a range of forest ecosystems. Proceedings of the National Academy of Sciences. doi.org/10.1073/pnas.2205682119.